1. Field of the Invention
The present invention relates to an improvement in spectral sensitizing methods for photographic light-sensitive emulsions, more particularly, it is concerned with a method of spectrally sensitizing photographic light-sensitive emulsions wherein effective spectral sensitization is achieved by improving the method of adding sensitizing dyes to the emulsions.
2. Description of the Prior Art
In the art of manufacturing photographic light-sensitive materials, it is well known that spectral sensitizing techniques (i.e., techniques for extending the spectral sensitization distribution of a light-sensitive material so as to range over the visible wavelength region) play an important roll, and they are essential for color photographic materials. Generally speaking, the process of spectrally sensitizing a photographic light-sensitive emulsion consists of dyeing light-sensitive materials dispersed therein, e.g., inorganic light-sensitive elements such as crystals of silver halide, zinc oxide, cadmium sulfide, titanium oxide or the like, and organic light-sensitive elements such as organic silver complexes, organic macromolecular photo-conductors or the like, by the addition of spectral sensitizing dyes suitable for each light-sensitive material. In particular, many methods have been introduced to the art for spectrally sensitizing silver halide photographic emulsions. The state of adsorption of a spectral sensitizing dye on the surfaces of silver halide crystals in an emulsion is influenced by the conditions of adding the dye, which has substantial effects on photographic characteristics such as fog, sensitivity, spectral sensitization distribution, desensitization, the stability of the sensitization, etc., as is described in detail in The Theory of the Photographic Process by C. E. K. Mees, 2nd Edition, Chapter 12, pages 430-500, Macmillan Publishers, 1954.
The most important condition which is required to provide a stable spectral sensitization action on light-sensitive crystals with the high sensitivity thereof retained is to achieve a state where each of the spectral sensitizing dye molecules which is added to an emulsion arrives at one of the adsorption sites of a light-sensitive crystal in a stable manner, to result in the adsorption thereof by interaction with a binder incorporated in the emulsion, and without aggregating or separating out of the binder. Moreover, another important condition is that the light-sensitive crystals dispersed in the light-sensitive emulsion should each homogeneously adsorb the same quantity of spectral sensitizing dye molecules.
A wide variety of methods for adding spectral sensitizing dyes to light-sensitive photographic emulsions are known. As disclosed in Japanese Patent Publication 22948/69, a solution prepared by dissolving a spectral sensitizing dye in a volatile, slightly water-soluble organic solvent is mixed with a hydrophilic colloid, followed by heating the mixture to remove the solvent therefrom to result in the formation of a dispersion. In this method, some spectral sensitizing dyes decompose during the heating which is necessary to remove the volatile organic solvent. In addition, this method has an economic disadvantage of requiring heating.
Moreover, there is another method wherein substantially water-insoluble spectral sensitizing dyes are dispersed in a water-soluble organic solvent without being dissolved therein. This method has the economic disadvantage that a long time is required to disperse such dyes using a colloid mill or the like.
In another commonly used method, a solution prepared by dissolving a dye in a water-miscible organic solvent (e.g., acetone, methanol, ethanol, propanol, methyl Cellosolve, pyridine, etc.) is added to an emulsion. Water may be added in part to these organic solvents, if desired. In such a method, high-speed coating (e.g., a coating speed of more than 100 meters per minute) is difficult, particularly when the solubility of the spectral sensitizing dye in the organic solvent is low, as the high amounts of organic solvent employed therein lower the surface active ability of coating aids therein, flocculate the binder and further solidify couplers present in the case of color light-sensitive materials. Often only a weak spectral sensitization can be obtained with such a method, because binder molecules are adsorbed by the spectral sensitizing dye molecules in preference to the silver halide crystals, or because the spectral sensitizing dye molecules themselves aggregate when the dye-containing solution is added to the emulsion as the organic solvent employed to dissolve the dye immediately mixes with water.
In addition to the above, a water solution of a spectral sensitizing dye as disclosed in Japanese Patent Publication 27555/69, dissolving a spectral sensitizing dye in a water solution of an anionic surface active agent as disclosed in U.S. Pat. No. 3,822,135 or using an acidic aqueous solution (made acidic by adding a strong acid to a spectral sensitizing dye-containing water solution) as disclosed in Japanese Patent Publication 23389/69 are known.
In the case of dissolving a dye in an acidic solution using a strong acid as disclosed in Japanese Patent Publication 23389/69, the resulting emulsion does become stable to light-exposure, but a number of dyes are hard to dissolve in an acidic aqueous solution, and even if certain dyes can be dissolved therein they may decompose upon storage of the solutions of the dyes. Further, even in the case of acidic aqueous solutions ranging in pH from pH 6.0 to pH 7.5 as disclosed in Japanese Patent Publication 27555/69, many dyes are hard to dissolve in aqueous solutions having such pH values, and many dissolvable dyes decompose because of poor storage capability with the passage of time. Therefore, a number of dyes cannot be employed therein.
Further, the method of solubilizing dyes in an aqueous solution containing an anionic surface active agent beyond the CMC concentration is useful for dissolving dyes slightly soluble in water, as disclosed in U.S. Pat. No. 3,822,135. However, there are many dyes which cannot be solubilized by such an anionic surface active agent-containing aqueous solution, so there are many cases in which water solutions of dyes cannot be formed.